1. Field of the Invention
This invention relates to surge arresters, and in particular to assemblies comprising a surge arrester that are arranged to provide an indication in the event of their failure.
2. Introduction to the Invention
Surge arresters, sometimes referred to as surge diverters, are devices arranged to protect other electrical equipment, usually in the distribution and supply of electric power, from an excessively high, and thus usually damaging, electric voltage, caused, for example, by a lightning strike. The surge arrester is electrically connected between the equipment at high voltage, say 5 kV or higher, and earth potential, and is electrically insulating in the absence of the overvoltage, that is to say a voltage in excess of that which can be withstood by the associated equipment. On occurrence of an overvoltage, the surge arrester becomes conductive so as safely to divert the consequent current to earth. The arrester then reverts to its insulating condition.
In some instances, however, a lightning strike may occur so close to a surge arrester that the electrical power that flows through the arrester is so intense as to damage it, in extreme cases totally destroying it. In less extreme cases, the damage may result in the formation of a continuous current path to earth. A surge arrester can also be subject to other fault conditions. Faults can arise in power distribution networks in which a relatively low fault current, of say 10 amps, flows for a relatively short time between high voltage and earth through the surge arrester. This can occur in particular in networks employing isolated neutral conductors, neutral conductors with impedance grounding, and those with uni-grounded neutral conductors associated with very high grounding resistance at a remote location. In such cases, the surge arrester can fail internally but with no external evidence of damage, so that visual identification of the failed arrester by service personnel is difficult.
Disconnectors are sometimes employed to isolate a failed arrester from the voltage applied thereto or from earth. The disconnector is arranged, upon detection of a predetermined fault condition, usually a flow of a minimum current for a minimum time, physically to separate from the surge arrester, thus interrupting the path to earth. An arc is initially drawn, but is extinguished as the disconnector moves further away. However, unless visual inspection of every surge arrester in the network is to be made after each temporary phase-to-ground fault, the network will continue to operate in a condition with a failed and disconnected arrester, and thus at a reduced safety level. This can arise because earth faults on an overhead power line, for example, are fairly common. The earth fault, that is to say the operation of the disconnector, is detected at the system control room and a circuit breaker switches off the power supply and after a short time switches the power on again. If the fault were a temporary one, the system would then continue in its powered state. The earth fault may have arisen at any part of the system, not at all necessarily associated with a surge arrester. Thus the fact that the fault has cleared provides no motivation for a linesman to be sent to check on the condition of all the surge arresters.
WO-A-93/01641 (Joslyn Corporation) discloses a high voltage surge arrester with a failed surge arrester signalling device. In this arrangement, if the arrester fails, the disconnector disengages and provides a visible indication that the arrester has failed, and at the same time the connection to the surge arrester is re-established by a further conductor, thereby to maintain a conductive path between the surge arrester and earth potential. Upon explosive separation of the disconnector from the arrester, the disconnector remains attached to the arrangement only by a flexible conductor, and forms no part of the re-connection circuit. The further conductor, which establishes the re-connection to the surge arrester, is formed from a resilient member that is released from tension by the departing disconnector and which establishes re-connection back to the surge arrester by means of its resilience. In some circumstances, such as flow of high current, which can generate high repulsive mechanical forces, such resilient force may not be sufficient to maintain good electrical connection.